Motility is not everything. Another major force that shapes cells is metabolism. Nutrients are often scarce, and increasing your cell’s ability to absorb them can give it a boost in the competitive game of life. So how can you do that? Remember that a sphere maximizes volume relative to surface area. To maximize surface area (for nutrient uptake) relative to volume, you would instead want something spikier. Some bacteria extend prosthecae (“add-ons” or appendages) for this purpose. Some, like Caulobacter crescentus, use a single prostheca, which is also called a stalk. Stalks are commonly located at the pole of the cell, where, as you’ll see in Chapter 8.4, they can help cells attach to a surface and hang on even in turbulent flow. Other species have a stalk on either end. Still others, like this Verrucomicrobium spinosum, form astral shapes with prosthecae jutting out in all directions.
Prosthecae offer an architectural challenge: thin extensions are delicate. Prosthecate cells use cytoskeletal proteins to form and stabilize their stalks, although exactly how this works remains unclear. One of these cytoskeletal proteins is Bactofilin (⇩), which is similar to the proteins that make intermediate filaments in eukaryotes. C. crescentus use Bactofilin polymers to help make their stalks. Prosthecobacter contain a different cytoskeletal element–microtubules–in their stalks, the function of which remains unclear (⇩).